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Ointments are semisolid preparations intended for external application to the skin or mucous membranes.
Ointment bases recognized for use as vehicles fall into four general classes: the hydrocarbon bases, the absorption bases, the water-removable bases, and the water-soluble bases. Each therapeutic ointment possesses as its base a representative of one of these four general classes.
Hydrocarbon Bases
These bases, which are known also as “oleaginous ointment bases,” are represented by White Petrolatum and White Ointment. Only small amounts of an aqueous component can be incorporated into them. They serve to keep medicaments in prolonged contact with the skin and act as occlusive dressings. Hydrocarbon bases are used chiefly for their emollient effects, and are difficult to wash off. They do not “dry out” or change noticeably on aging.
Absorption Bases
This class of bases may be divided into two groups: the first group consisting of bases that permit the incorporation of aqueous solutions with the formation of a water-in-oil emulsion (Hydrophilic Petrolatum and Lanolin), and the second group consisting of water-in-oil emulsions that permit the incorporation of additional quantities of aqueous solutions (Lanolin). Absorption bases are useful also as emollients.
Water-Removable Bases
Such bases are oil-in-water emulsions, e.g., Hydrophilic Ointment, and are more correctly called “creams.” (See Creams.) They are also described as “water-washable,” since they may be readily washed from the skin or clothing with water, an attribute that makes them more acceptable for cosmetic reasons. Some medicaments may be more effective in these bases than in hydrocarbon bases. Other advantages of the water-removable bases are that they may be diluted with water and that they favor the absorption of serous discharges in dermatological conditions.
Water-Soluble Bases
This group of so-called “greaseless ointment bases” comprises water-soluble constituents. Polyethylene Glycol Ointment is the only Pharmacopeial preparation in this group. Bases of this type offer many of the advantages of the water-removable bases and, in addition, contain no water-insoluble substances such as petrolatum, anhydrous lanolin, or waxes. They are more correctly called “Gels.” (See Gels.)
Choice of Base— The choice of an ointment base depends upon many factors, such as the action desired, the nature of the medicament to be incorporated and its bioavailability and stability, and the requisite shelf-life of the finished product. In some cases, it is necessary to use a base that is less than ideal in order to achieve the stability required. Drugs that hydrolyze rapidly, for example, are more stable in hydrocarbon bases than in bases containing water, even though they may be more effective in the latter.